ui: do not use rawinput() when we have to replace sys.stdin/stdout

ui: do not use rawinput() when we have to replace sys.stdin/stdout

See the inline comment for why. The current Python3 hack doesn't work if
more than one user inputs are expected because TextIOWrapper fills its
internal buffer at the first read() request.

Maybe we could write an unbuffered TextIOWrapper, but I don't want to make
things more complicated. Instead, this patch reinvents raw_input(' ') of
no readline support.

--- a/contrib/python3-whitelist	Tue Mar 06 02:32:26 2018 -0600
+++ b/contrib/python3-whitelist	Tue Mar 06 02:38:53 2018 -0600
@@ -3,6 +3,7 @@
 test-addremove-similar.t
 test-addremove.t
 test-amend-subrepo.t
+test-amend.t
 test-ancestor.py
 test-annotate.py
 test-annotate.t
@@ -185,6 +186,8 @@
 test-largefiles-cache.t
 test-largefiles-misc.t
 test-largefiles-small-disk.t
+test-largefiles-update.t
+test-lfs-largefiles.t
 test-locate.t
 test-lock-badness.t
 test-log.t
@@ -196,11 +199,13 @@
 test-manifest.t
 test-match.py
 test-mdiff.py
+test-merge-changedelete.t
 test-merge-closedheads.t
 test-merge-commit.t
 test-merge-criss-cross.t
 test-merge-default.t
 test-merge-force.t
+test-merge-halt.t
 test-merge-internal-tools-pattern.t
 test-merge-local.t
 test-merge-remove.t
@@ -228,11 +233,13 @@
 test-mq-qimport-fail-cleanup.t
 test-mq-qpush-exact.t
 test-mq-qqueue.t
+test-mq-qrefresh-interactive.t
 test-mq-qrefresh-replace-log-message.t
 test-mq-qrefresh.t
 test-mq-qrename.t
 test-mq-qsave.t
 test-mq-safety.t
+test-mq-subrepo.t
 test-mq-symlinks.t
 test-mv-cp-st-diff.t
 test-narrow-archive.t

--- a/mercurial/ui.py	Tue Mar 06 02:32:26 2018 -0600
+++ b/mercurial/ui.py	Tue Mar 06 02:38:53 2018 -0600
@@ -1264,6 +1264,10 @@
         return i
 
     def _readline(self):
+        # Replacing stdin/stdout temporarily is a hard problem on Python 3
+        # because they have to be text streams with *no buffering*. Instead,
+        # we use rawinput() only if call_readline() will be invoked by
+        # PyOS_Readline(), so no I/O will be made at Python layer.
         usereadline = (self._isatty(self.fin) and self._isatty(self.fout)
                        and util.isstdin(self.fin) and util.isstdout(self.fout))
         if usereadline:
@@ -1280,13 +1284,16 @@
         # prompt ' ' must exist; otherwise readline may delete entire line
         # - http://bugs.python.org/issue12833
         with self.timeblockedsection('stdio'):
-            sin, sout = sys.stdin, sys.stdout
-            try:
-                sys.stdin = encoding.strio(self.fin)
-                sys.stdout = encoding.strio(self.fout)
+            if usereadline:
                 line = encoding.strtolocal(pycompat.rawinput(r' '))
-            finally:
-                sys.stdin, sys.stdout = sin, sout
+            else:
+                self.fout.write(b' ')
+                self.fout.flush()
+                line = self.fin.readline()
+                if not line:
+                    raise EOFError
+                if line.endswith(pycompat.oslinesep):
+                    line = line[:-len(pycompat.oslinesep)]
 
         # When stdin is in binary mode on Windows, it can cause
         # raw_input() to emit an extra trailing carriage return